1. Field of the Invention
This invention is concerned with analysis of reducing agents in electroless plating baths as a means of providing control over the deposit properties.
2. Description of the Related Art
Plating baths are widely used by the electronics industry to deposit a variety of metals (copper, nickel and gold, for example) on various parts, including circuit boards, semiconductor chips, and device packages. Both electroplating baths and electroless plating baths are employed. For electroplating, the part and a counter electrode are brought into contact with the electroplating bath containing ions of an electrodepositable metal, and the metal is electrodeposited by applying a negative potential to the part relative to the counter electrode. For electroless plating, the bath also contains a reducing agent which, in the presence of a catalyst, chemically reduces the metal ions to form a deposit of the metal. Since the deposited metal itself may serve as the catalyst, the electroless deposition, once initiated, proceeds without the need for an externally applied potential.
Electroplating baths typically contain organic additives whose concentrations must be closely controlled in the low parts per million range in order to attain the desired deposit properties and morphology. One of the key functions of such additives is to level the deposit by suppressing the electrodeposition rate at protruding areas in the substrate surface and/or by accelerating the electrodeposition rate in recessed areas. Accelerated deposition may result from mass-transport-limited depletion of a suppressor additive species that is rapidly consumed in the electrodeposition process, or from accumulation of an accelerating species that is consumed with low efficiency. The most sensitive methods available for detecting leveling additives in plating baths involve electrochemical measurement of the metal electrodeposition rate under controlled hydrodynamic conditions, for which the additive concentration in the vicinity of the electrode surface is well-defined.
Cyclic voltammetric stripping (CVS) analysis [D. Tench and C. Ogden, J. Electrochem. Soc. 125, 194 (1978)] is the most widely used bath additive control method and involves cycling the potential of an inert electrode (e.g., Pt) in the plating bath between fixed potential limits so that metal is alternately plated on and stripped from the electrode surface. Such potential cycling is designed to establish a steady state for the electrode surface so that reproducible results are obtained. Accumulation of organic films or other contaminants on the electrode surface can be avoided by periodically cycling the potential of the electrode in the plating solution without organic additives and, if necessary, polishing the electrode using a fine abrasive. Cyclic pulse voltammetric stripping (CPVS), also called cyclic step voltammetric stripping (CSVS), is a variation of the CVS method that employs discrete changes in potential during the analysis to condition the electrode so as to improve the measurement precision [D. Tench and J. White, J. Electrochem. Soc. 132, 831 (1985)]. A rotating disk electrode configuration is typically employed for both CVS and CPVS analysis to provide controlled hydrodynamic conditions.
For CVS and CPVS analyses, the metal deposition rate may be determined from the current or charge passed during metal electrodeposition but it is usually advantageous to measure the charge associated with anodic stripping of the metal from the electrode. A typical CVS/CPVS rate parameter is the stripping peak area (Ar) for a predetermined electrode rotation rate. The CVS method was first applied to control copper pyrophosphate baths (U.S. Pat. No. 4,132,605 to Tench and Ogden) but has since been adapted for control of a variety of other plating systems, including the acid copper sulfate baths that are widely used by the electronics industry [e.g., R. Haak, C. Ogden and D. Tench, Plating Surf. Fin. 68(4), 52 (1981) and Plating Surf. Fin. 69(3), 62 (1982)].
Acid copper sulfate baths are employed in the xe2x80x9cDamascenexe2x80x9d process (e.g., P. C. Andricacos, Electrochem. Soc. Interface, Spring 1999, p.32; U.S. Pat. No. 4,789,648 to Chow et al.; U.S. Pat. No. 5,209,817 to Ahmad et al.) to electrodeposit copper within fine trenches and vias in dielectric material on semiconductor chips. CVS methods for controlling the three organic additives in acid copper baths needed for plating ultra-fine Damascene features are described in U.S. patent application Ser. No. 09/968,202 to Chalyt et al. (filed Oct. 1, 2001), now U.S. Pat. No. 6,572,753. In the Damascene process, as currently practiced, vias and trenches are etched in the chip""s dielectric material, which is typically silicon dioxide, although materials with lower dielectric constants are under development. A barrier layer, e.g., titanium nitride (TiN), tantalum nitride (TaN) or tungsten nitride (WNX), is deposited on the sidewalls and bottoms of the trenches and vias, typically by reactive sputtering, to prevent Cu migration into the dielectric material and degradation of the device performance. Over the barrier layer, a thin copper seed layer is deposited, typically by sputtering, to provide enhanced conductivity and good adhesion. Copper is then electrodeposited into the trenches and vias. Copper deposited on the outer surface, i.e., outside of the trenches and vias, is removed by chemical mechanical polishing (CMP). A capping or cladding layer (e.g., TiN, TaN or WNX) is applied to the exposed copper circuitry to suppress oxidation and migration of the copper. The xe2x80x9cDual Damascenexe2x80x9d process involves deposition in both trenches and vias at the same time. In this document, the term xe2x80x9cDamascenexe2x80x9d also encompasses the xe2x80x9cDual Damascenexe2x80x9d process.
Damascene barrier layers based on electrolessly deposited cobalt and nickel are currently under investigation [e.g., Kohn et al., Mater. Sci. Eng. A302, 18 (2001)]. Such metallic materials have higher electrical conductivities compared to metal nitride barrier materials, which enables copper to be electrodeposited directly on the barrier layer without the use of a copper seed layer. Higher barrier layer conductivity also reduces the overall resistance for circuit traces of a given cross-sectional area. In addition, electroless deposition provides, highly conformal coatings, even within ultra-fine trenches and vias, so that the overall coating thickness can be minimized. Electroless cobalt and nickel baths being investigated for Damascene barrier deposition typically also contain a refractory metal (e.g., tungsten, molybdenum or rhenium), which co-deposits with the cobalt or nickel and increases the maximum temperature at which effective barrier properties are retained.
For electroless cobalt and nickel baths, hypophosphite (H2PO2) is typically used as the reducing agent, which introduces phosphorus into the deposit. The codeposited phosphorus reduces the deposit grain size and crystallinity (compared to electrodeposits), which tends to improve the deposit barrier properties. Alternative reducing agents include the boranes, dimethylamineborane (DMAB), for example. Use of a borane reducing agent introduces boron into the deposit. A typical bath for electroless deposition of Damascene barrier layers comprises 0.1 M cobalt chloride or sulfate, 0.2 M sodium hypophosphite, 0.03 M sodium tungstate, 0.5 M sodium citrate, 0.5 M boric acid, and a small amount of a surfactant. Such Co(W,P) baths typically operate at about pH 9 and a temperature of 85xc2x0-95xc2x0 C., and may also contain organic additives.
For electroless deposition of cobalt and nickel on dielectric materials, such as silicon oxide, or on metals that are not sufficiently catalytic for the electroless process, such as copper, a seed layer of a catalytic metal is generally employed. Typically, catalytic palladium is deposited by immersion of the part in an acidic activator solution containing palladium chloride and fluoride ion. The fluoride ion tends to cause dissolution of surface oxides on the substrate so that a displacement layer of palladium is formed. Alternatively, a seed layer of the electrolessly deposited metal, cobalt or nickel, may be applied by sputtering.
Recently, direct deposition of capping layers of Co(W,P) on Damascene copper circuits was reported (T. Itabashi, N. Nakano and H. Akahoshi, Proc. IITC 2002, p. 285-287) for a Co(W,P) bath employing two reducing agents. In this case, electroless deposition is initiated by the more active reducing agent (DMAB), which is present at a relatively low concentration. As the DMAB reducing agent becomes depleted at the part surface, electroless deposition is sustained by the less active reducing agent (hypophosphite), which provides better deposit properties.
Close control of the concentrations of reducing agents in electroless plating baths is necessary to provide acceptable deposit properties but available reducing agent analysis methods are cumbersome and inadequate. In a typical prior art method, the reducing agent in a plating bath sample is first fully oxidized in an acidic solution by addition of excess iodine. This oxidation reaction requires about 30 minutes and must be performed in the absence of light. The excess iodine in the acidic solution is then back-titrated with a solution containing thiosulfate ion, typically using loss of solution color as the titration endpoint. Such prior art methods do not provide analysis results within the time frame needed for close control of the reducing agent, and are not amenable to automated on-line bath control.
This invention provides a method for determining the concentration of a reducing agent in an electroless plating bath from the increase produced by the reducing agent in the electrodeposition rate of a metal. The metal electrodeposition rate is measured for a test solution comprising an electrodeposition solution and a known volume fraction of the electroless plating bath, and for at least two calibration solutions containing known concentrations of the reducing agent in the electrodeposition solution. One of the calibration solutions may be the electrodeposition solution without added reducing agent. The metal electrodeposited from the electrodeposition solution may be the same metal as the electrolessly deposited metal or may be a different metal. The reducing agent concentration in the electroless plating bath is determined by comparing the metal electrodeposition rate for the test solution with the metal electrodeposition rate measured for the calibration solutions.
In a preferred embodiment, the concentration of a reducing agent in a plating bath for electroless plating of a first metal is determined from the increase produced by the reducing agent in the electrodeposition rate of a second metal in an electrodeposition solution. A calibration curve is generated by measuring an electrodeposition rate parameter for the second metal in the electrodeposition solution containing known concentrations of the reducing agent. Two calibration solutions are needed, one of which may be the electrodeposition solution without reducing agent. For the reducing agent analysis, the electrodeposition rate parameter is measured for a test solution containing a known volume fraction of the plating bath sample added to the electrodeposition solution, which may initially contain no reducing agent or a relatively small concentration of reducing agent. Preferably, the rate parameters for the calibrations and the reducing agent analysis are normalized with respect to the electrodeposition rate parameter for the electrodeposition solution containing little or no reducing agent. Metal electrodeposition rates are preferably determined from the current or charge associated with voltammetric plating and stripping of the second metal at a rotating disk electrode comprised of an inert metal (e.g., platinum). In this case, the electrodeposition solution is chosen to provide reversible electrodeposition of the second metal.
The method of the present invention is particularly useful for measuring the concentration of reducing agents in electroless cobalt and nickel baths of the type used for depositing barrier layers for Damascene copper circuits, for example. In a preferred approach, measurements of the copper electrodeposition rate in an acid copper sulfate electrodeposition solution (preferably without organic additives) are used to determine the concentration of the reducing agent in the electroless cobalt or nickel plating bath. A preferred electrodeposition rate parameter is the CVS peak area (Ar) measured at a platinum disk electrode rotating at constant rate. This approach may also be used to measure the reducing agent concentration for cobalt and nickel electroless plating baths involving co-deposition of other metals (tungsten, molybdenum or rhenium, for example).
The method of the present invention may also be used to measure the concentrations of individual reducing agents in electroless plating baths employing more than one reducing agent. For example, hypophosphite and dimethylamineborane (DMAB) reducing agents in electroless cobalt and nickel baths may be analyzed by taking advantage of the instability of DMAB (compared to hypophosphite) in acidic solutions.
The present invention provides an analysis method that enables reducing agents in electroless plating baths to be analyzed and controlled so as to ensure acceptable metal deposits. This method requires relatively few chemical reagents, avoids the complicated procedures of prior art methods, and can be performed rapidly, which enables close process control. The only sample preparation needed is dilution with de-ionized water or a predetermined solution (an acidic solution, for example). The method may be used for analysis of electroless plating baths for deposition of a variety of metals and alloys, including those which tend to form passive oxide layers.